Ultrashort Pulses

Ultrashort pulses are pulses of light that have a duration of picoseconds or less. The light pulses generated in mode-locked lasers can be very short, especially for passive mode-locking. There is no accepted definition of "ultra-short", but in general, this label applies to pulses if they are lasting up to a few tens of picoseconds and are often in the femtosecond range. Note that ultrashort pulses should not be called "ultrafast pulses", they are no faster than long pulses (no higher speeds).

Generation of ultrashort pulses

Ultrashort pulses are typically generated by passively mode-locked lasers, but are sometimes also generated by optical parametric amplifiers (possibly using a supercontinuum as input) or free-electron lasers. It is also possible to start with longer pulses and apply some method of pulse compression, but in most cases the input pulses of this compression are already ultrashort pulses.

Optical bandwidth

Essentially, ultrashort pulses have a wide optical bandwidth. Even if their instantaneous frequencies are nearly constant throughout the duration of the pulse, the width of the spectrum is at least an order of magnitude of the inverse pulse duration. This is mainly because, for some medium of resonant excitation, it only makes sense when the change in the optical frequency is large enough to induce a significant phase change over the duration of the pulse.

In many cases, the instantaneous frequency of the pulse is approximately constant, and the time-bandwidth product is slightly less than 1. A non-chirped Gaussian pulse with a pulse duration of 1ps (full width at half height) has an optical bandwidth of ≈0.44 THz. Very short pulses (few-period pulses) can have octave span spectra.

Shaping ultrashort pulses

When ultrashort pulses are generated in mode-locked lasers, they can be affected by various pulse shaping phenomena. For example, there is usually a saturable absorber, which will have high losses at the beginning or even at the end of the pulse. In addition to lasers, ultrashort pulses can be further manipulated by various pulse shapers.

Characterization of ultrashort pulses

There are several methods for pulse characterization. While some methods only allow the measurement of basic pulse parameters, such as pulse duration, others can be used for "complete" characterization, i.e., to obtain the entire electric field and spectral phase over time. The results can be visualized in a variety of ways, for example, with a time- or frequency-dependent function plot, or with a spectrogram. Most commonly, pulse duration is measured using an autocorrelator.